The invention relates to a collecting main for the discharge of hot process gases from tubular cracking furnaces as frequently used in natural gas reforming plants for the production of synthesis gas. Such collecting main is always provided with several nozzles via which the process gas tubes of the tubular cracking furnace can be connected to the collecting main. In the area of the nozzles, the process gas tubes usually run in guide sleeves. The gas outlets of the guide sleeves or process gas tubes end in the collecting main and extend into it. Under proper operating conditions the reaction gas is introduced into the collecting main via these gas outlets, the present invention providing at least part of the outlets in the form of pipe bends. Furthermore, the invention relates to the use of such collecting mains in a tubular cracking furnace.
A plurality of tubular cracking furnaces is known which are used for the indirect heating of crackable fluids, especially for the cracking of hydrocarbons for the production of synthesis gas and/or hydrogen. In these tubular cracking furnaces a plurality of process gas tubes are arranged in the furnace chamber in the form of registers or bundles, these process gas tubes being connected to collecting mains running underneath the furnace bottom. Patent documents DE 1 542 530 B or DE 10 2006 022898 describe such tubular cracking furnaces.
As further described in DE 1 542 530 B or DE 10 2006 022898, the collecting mains consist of several layers, i.e. of at least an outer metal jacket tube, the so-called pressure jacket, which is usually made of weldable steel material and an inner insulation tube usually made of refractory brick-lining or refractory concrete. DE 1 667 324 B describes a three-layer tube as used for industrial applications. The innermost layer may consist of a metal inner tube as already suggested in DE 1 542 530 B for the area where the process gas tube penetrates into the collecting main. This inner tube serves to protect the concrete insulation from erosion caused by the process gas flow. The temperature in the collecting main ranges between approx. 800° C. and 950° C.
The thickness or insulation efficiency of the insulating layer is to be selected such that it protects the steel material of the outer or jacket tube by limiting the temperature in the outer or jacket tube to approx. 200° C. on the one hand and on the other hand by preventing the concrete from becoming too cold. Usually the stress acting upon the concrete as a result of thermal expansion processes increases the porosity of the concrete which leads to an escape of process gas from the metal inner tube which is generally not of gas-proof design. These small gas flows are also called creep flows which may convey considerable amounts of heat to the outer jacket tube where they produce local overheating.